LIFT SYSTEM

Abstract

A hydraulic lift system comprises a plurality of hydraulic cylinders with pistons therein, having piston rods that are mechanically interconnected so that the pistons and piston rods move upwards and downwards in unison. A hydraulic fluid communication sub-system is operable to deliver fluid from a source of pressurized hydraulic fluid to at least a first cylinder to drive the pistons through an upstroke, from the source of pressurized hydraulic fluid to a second cylinder to drive the pistons through a downstroke, and from the first cylinder to the second cylinder. A hydraulic fluid flow control system selectively directs fluid from the source of pressurized hydraulic fluid either to the first cylinder to drive the pistons upwardly or to the second cylinder to drive the pistons downwardly. During the downstroke, hydraulic fluid flow control system directs hydraulic fluid from the first cylinder to the second cylinder help drive the pistons downwardly.

Description

TECHNICAL FIELD

The present invention relates to lift systems, including hydraulic lift systems used in pump jack applications.

BACKGROUND

Hydraulic lift systems are used in a number of applications. One type of application is as a pump jack for operating a down-well pump. Hydraulic lift systems used in this type of application are an alternative to conventional “donkey” or “rocking-arm” type pump jacks.

Typically, hydraulic lift systems which are used as pump jacks suffer from a number of problems. These problems may include complexity, low efficiency and high power requirements. Often, the shortcomings of current hydraulic lift systems makes them unsuitable for use as pump jacks other than in temporary production or tuning applications.

Accordingly, there is a need for improved lift systems.

SUMMARY

In an aspect of the present invention, there is provided a hydraulic lift system comprising: a source of pressurized hydraulic fluid; a plurality of hydraulic cylinders, each one of the cylinders having a piston therein with a piston rod of the piston extending from an end of each one of the hydraulic cylinders, wherein the piston rods are mechanically interconnected so that the piston rods and the pistons of each of the hydraulic cylinders are operable to move upwards and downwards in unison with each other; a hydraulic fluid communication sub-system operable to deliver fluid from the source of pressurized hydraulic fluid to at least a first cylinder of the plurality of hydraulic cylinders to drive the pistons in an upward direction through an upstroke; the hydraulic fluid communication sub-system also operable to deliver hydraulic fluid from the source of pressurized hydraulic fluid to a second cylinder of the plurality of hydraulic cylinders to drive the pistons in a downward direction through a downstroke; the hydraulic fluid communication sub-system also operable to deliver hydraulic fluid from the first cylinder to the second cylinder; a hydraulic fluid flow control sub-system operable to: (a) selectively direct hydraulic fluid from the source of pressurized hydraulic fluid to the first cylinder to drive the pistons in an upward direction to provide an upstroke; and (b) alternatively, selectively direct hydraulic fluid from the source of pressurized hydraulic fluid to the second cylinder to drive the pistons in a downward direction to provide a downstroke; and (c) during the downstroke direct hydraulic fluid from the first cylinder to the second cylinder to assist the second cylinder in driving the pistons in the downward direction during the downstroke.

In another aspect of the present invention, there is provided a method of reciprocating a down-well pump in a shaft of a well, the method comprising: a) pumping a pressurized fluid into a lift chamber of a first hydraulic cylinder to lift a carriage coupled to the down-well pump and to a piston of the first hydraulic cylinder; b) pumping a pressurized fluid into a lowering chamber of a second hydraulic cylinder having a piston coupled to the carriage, to lower the carriage; c) connecting the lift chamber in fluid communication with the lowering chamber such that pressurized fluid is expelled from the lift chamber into the lowering chamber during the lowering.

In another aspect of the present invention, there is provided a lift system comprising a pump for supplying a flow of pressurized driving fluid; at least one upward driving cylinder having a movable piston rod; at least one downward driving cylinder having a movable piston rod; the piston rods of the upward driving cylinder and the downward driving cylinder being interconnected to each other such that the piston rods of both the upward driving cylinder and the downward driving cylinder are operable to move upwards and downwards in unison with each other; a driving fluid communication sub-system operable to deliver a flow of driving fluid supplied by the pump from the pump to the upward driving cylinder to drive the piston rods in an upward direction in an upstroke; the driving fluid communication sub-system also operable to deliver a flow of driving fluid supplied by the pump from the pump to the downward driving cylinder to drive the piston rods in a downward direction in a downstroke; and the driving fluid communication sub-system also operable to deliver a flow of driving fluid in the upward driving cylinder from the upward driving cylinder to the downward driving cylinder during the downstroke; a fluid direction control sub-system operable to: (a) in a first mode of operation to direct a flow of driving fluid from the pump to the upward driving cylinder to drive the pistons in an upward direction to create an upstroke; (b) in a second mode of operation to direct a flow of driving fluid from the pump to the downward driving cylinder to drive the pistons in a downward direction to create a downstroke; and (c) in the second mode of operation, to also direct a flow of driving fluid from the upward driving cylinder to the downward driving cylinder during the downstroke, such that during the downstroke, the driving fluid is delivered from the upward driving cylinder to/towards the downward driving cylinder to assist the downward driving cylinder in driving the pistons in the downward direction during the downstroke.

In another aspect of the present invention, there is provided a method of moving a reciprocating mass upwards and downwards, the method comprising a) providing a pump for supplying a flow of pressurized driving fluid; b) providing at least one upward driving cylinder having a movable piston rod interconnected to the reciprocating mass; c) providing at least one downward driving cylinder having a movable piston rod interconnected to the reciprocating mass; the piston rods of the upward driving cylinder and the downward driving cylinder being interconnected to each other such that the piston rods of both the upward driving cylinder and the downward driving cylinder are operable to move upwards and downwards in unison with each other; d) providing a driving fluid communication sub-system for delivering a flow of driving fluid supplied by the pump from the pump to the upward driving cylinder to drive the piston rods in an upward direction in an upstroke and for delivering a flow of driving fluid supplied by the pump from the pump to the downward driving cylinder to drive the piston rods in a downward direction in a downstroke, the driving fluid communication sub-system also for delivering a flow of driving fluid in the upward driving cylinder from the upward driving cylinder towards the downward driving cylinder during the downstroke; e) providing a fluid direction control sub-system; f) directing a flow of driving fluid from the pump to the upward driving cylinder to drive the pistons in an upward direction to create an upstroke to thereby move the reciprocating mass upwards; g) directing a flow of driving fluid from the pump to the downward driving cylinder to drive the pistons in a downward direction to create a downstroke; and simultaneously also directing a flow of driving fluid from the upward driving cylinder to the downward driving cylinder during the downstroke, such that during the downstroke, the driving fluid is delivered from the upward driving cylinder to the downward driving cylinder to assist the downward driving cylinder in driving the pistons in the downward direction during the downstroke, to thereby move the reciprocating mass downwards.

In another aspect of the present invention, there is provided a method of operating a lift system, wherein lift system comprises: a pump for supplying a flow of pressurized driving fluid; at least one upward driving cylinder having a movable piston rod; at least one downward driving cylinder having a movable piston rod; the piston rods of the upward driving cylinder and the downward driving cylinder being interconnected to each other such that the piston rods of both the upward driving cylinder and the downward driving cylinder are operable to move upwards and downwards in unison with each other; a driving fluid communication sub-system for delivering a flow of driving fluid supplied by the pump from the pump to the upward driving cylinder to drive the piston rods in an upward direction in an upstroke and for delivering a flow of driving fluid supplied by the pump from the pump to the downward driving cylinder to drive the piston rods in a downward direction in a downstroke, the driving fluid communication sub-system also for delivering a flow of driving fluid in the upward driving cylinder from the upward driving cylinder towards the downward driving cylinder during the downstroke; a fluid direction control sub-system operable to: (a) in a first mode of operation to direct a flow of driving fluid from the pump to the upward driving cylinder to drive the pistons in an upward direction to create an upstroke; (b) in a second mode of operation to direct a flow of driving fluid from the pump to the downward driving cylinder to drive the pistons in a downward direction to create a downstroke; and (c) in the second mode of operation, to also direct a flow of driving fluid from the upward driving cylinder to the downward driving cylinder during the downstroke; and wherein the method comprises: a) directing a flow of driving fluid from the pump to the upward driving cylinder to drive the pistons in an upward direction to create an upstroke; b) directing a flow of driving fluid from the pump to the downward driving cylinder to drive the pistons in a downward direction to create a downstroke; and simultaneously also directing a flow of driving fluid from the upward driving cylinder to the downward driving cylinder during the downstroke, such that during the downstroke, the driving fluid is delivered from the upward driving cylinder to the downward driving cylinder to assist the downward driving cylinder in driving the pistons in the downward direction during the downstroke.

BRIEF DESCRIPTION OF THE DRAWINGS

In the figures, which illustrate by way of example only, embodiments of this invention:

FIG. 1. is a schematic view of a lift system in exemplary of an embodiment of the present invention;

FIG. 2, is an enlarged front elevation view of a portion of the lift system of FIG. 1;

FIG. 3 is a schematic view of part of the lift system of FIG. 1;

FIG. 4 is a schematic view of the lift system of FIG. 1 in a stationary state;

FIG. 6 is a schematic view of forces acting on components of the lift system of FIG. 1 in the state of FIG. 4;

FIG. 6 is a schematic view of the lift system of FIG. 1 in a first state of operation;

FIG. 7 is a schematic view of forces acting on components of the lift system of FIG. 1 in the state of FIG. 6;

FIG. 8 is a schematic view of the lift system of FIG. 1 in a second state of operation;

FIG. 9 is a schematic view of forces acting on components of the lift system of FIG. 1 in the state of FIG. 8;

FIG. 10 is a schematic view of another lift system;

FIG. 11 is a schematic view of the lift system of FIG. 10 in a first state of operation;

FIG. 12 is a schematic view of the lift system of FIG. 10 in a second state of operation.

DETAILED DESCRIPTION

FIG. 1 depicts an example lift system 100. Lift system 100 may be installed at a wellhead 102 for extracting fluids, e.g. oil, natural gas and/or dewatering fluid from a reservoir 104.

Extraction of fluids from a reservoir 104 may be effected by operation of a down-well pump 106 at the bottom of a well shaft 108. Down-well pump 106 may be operated by up-and-down reciprocating motion of a sucker rod 110. It should be noted that in some applications, the well shaft 108 may not be oriented entirely vertically, but may have horizontal components and/or portions to its path.

With each downward stroke of sucker rod 110, down-well pump 106 may be moved downwardly and a one-way valve 112 opens, admitting fluid from reservoir 104 into down-well pump 106. During this downstroke, one-way valve 114 at the bottom of well shaft 108 may be closed, preventing fluids from escaping. During each upstroke of sucker rod 110, down-well pump 106 may be drawn upwardly and one-way valve 112 may be closed. Thus, fluids drawn in through one-way valve 112 during the downstroke can be raised. When one-way valve 114 opens, fluids can enter well shaft 108 through one-way valve 114 and passages 116. Successive upstrokes of down-well pump 106 form a column of fluid in well shaft 108 above down-well pump 106. Once this column of fluid is formed, each upstroke pushes a volume of fluid to the surface.

Sucker rod 110 may be actuated by a set of cylinders which may be hydraulic cylinders 118a, 118b, 118c (collectively, cylinders 118). Hydraulic cylinders 118 may be supported on a frame 120 mounted to well head 102 by conventional means, e.g., by welding and/or using fasteners. Sucker rod 110 may be attached to cylinders 118 by a carriage 122, described in more detail below. Cylinders 118 may be arranged above and in generally parallel orientation with sucker rod 110. Cylinder 118c may be vertically oriented and axially aligned with sucker rod 110 and cylinders 118a, 118b may be vertically oriented but transversely spaced equal distances to either side of cylinder 118c. Cylinders 118a-c may also be transversely aligned with each other in a transverse plane. That way, the forces acting up and down on carriage 122 by the cylinders 118 in both the upstroke and downstrokes of the lift system 100 may not cause any moment or rotation of the carriage about the axis of the sucker rod (ie. it will move the sucker rod up and down without any significant tendency to rotate the sucker rod).

Cylinders 118 may be powered by a fluid circuit 124. Fluid circuit 124 may comprise a reservoir 126 containing a driving fluid, such as hydraulic fluid, a source of pressurized driving fluid, such as pump 128 and a fluid communication sub-system comprising communication lines 130, 132, in fluid communication with cylinders 118 by way of a fluid flow control subsystem 134 that may comprise a plurality of valves as described below (also referred to herein as valve subsystem 134″). The driving fluid may for example be any suitable fluid that is substantially incompressible, contains anti-wear additives or constituents, and has an ability to transfer heat from within fluid circuit 124 to reservoir 126. By way of example, driving fluid within fluid circuit 124 may reach temperatures within the range of −40° C. to 80° C. Some example driving fluids include SKYDROL™ airplane fluid, automatic transmission fluid, and other synthetic and semi-synthetic fluids.

In the depicted embodiment, line 130 may be a hose or pipe with an internal diameter (ID) of 1 inch and line 132 may be a hose or pipe with an ID of 1.25 inches. Fluid communication lines described herein may be, for example, steel lines or steel braided hydraulic lines with appropriate pressure rating and resistance to environmental factors such as UV exposure, high temperature and abrasion. Pump 128 may be a variable-displacement piston pump able to deliver a flow rate of about 46 gallons of hydraulic fluid per minute at a pressure of at least 3000 psi. For example pump 128 may be a series 45 axial piston open circuit pump made by Sauer Danfoss. The output flow rate of a variable-displacement piston pump may be adjustable by changing the pump's displacement in each cycle. The flow rate of pump 128 determines the speed at which lift system 100 performs each downstroke or upstroke. Thus, conveniently, pump 128 may allow the operating speed, that is the speed and frequency of strokes of lift system 100, to be changed. By way of example only, lines which carry peak system hydraulic pressure may be sized to create a maximum fluid velocity of about 20 feet per second. Lines which carry low hydraulic pressure under 500 psi, that is, lines which drain to reservoir 126, may be sized to create a maximum fluid velocity of about 4 feet per second. Lines which carry counterbalance fluid may be sized to create a maximum fluid velocity of about 100 feet per second.

Pump 128 may be controlled for pressure-compensated operation. That is, pump 128 may be controlled to operate so as to maintain a substantially constant pressure of driving fluid. Run in these conditions, pump 128 may output a substantially constant volumetric flow rate of driving fluid. The pressure and flow rate output by pump 128 may be linked. That is, increasing the pressure produced by pump 128 may also result in an increased flow rate, while decreasing the pressure may result in a decreased flow rate. Also, since the forces acting upon the components of the lift system may be dynamic and vary over time, it is possible that in order to maintain a substantially constant rate of flow of driving fluid throughout the fluid circuit 124, the pump pressure may have to be adjusted. For example, if varying amounts of friction are encountered throughout the operation of the cylinders 118, the pressure setting of the pump may need to be adjusted. Further details of an example of suitable pump 128 for use with hydraulic fluid and the manner in which it is controlled may be found in a brochure entitled “Series 45 Axial Piston Open Circuit Pumps Technical Information”, published 2011 by Sauer Danfoss and available at http://www.sauer-danfoss.com/Products/PistonPumpsandMotors/OpenCircuitAxialPistonPumps/index.htm, the contents of which are hereby incorporated by reference.

In other embodiments, pump 128 could be another type of pump operable to deliver a suitable pressure and flow rate, such as a vane or gear pump with means of creating variable displacement and pump controls or external valves to regulate flow. Pump 128 may be configured to deliver a flow rate of about 46 gallons per minute (GPM), however in other embodiments, pump 128 may be sized to deliver flow rates between 10-150 GPM at pressures between 500-5000 psi. Pump 128 may be selected so that it can deliver a substantially constant flow rate during operation. In some embodiments, pump 128 may deliver a varying flow rate during operation.

Cylinders 118 may be provided with a counterbalance subsystem that can be used to offset some or all of the weight of the various components acting down. The counterbalance subsystem may comprise a counterbalance fluid reservoir 136 holding a counterbalance fluid. Counterbalance fluid reservoir 136 may be in fluid communication with a lower chamber of cylinder 118c through fluid communication line 138 and multi-way valve subsystem 134. The counterbalance fluid may be a compressible gas, and the gas may be inert. For example the counterbalance fluid may be nitrogen.

Turning to FIG. 2, lift system 100 is depicted in more detail. Cylinders 118 may be oriented such that they are aligned with one another in a transverse plane when connected to carriage 122. Carriage 122 may be a generally flat plate made from a suitably strong material such as steel. Cylinders 118 may be fixedly mounted to frame 120 by conventional attachment devices such as for example bolts and/or welding and may be oriented with their rod ends downwards. Cylinders 118 may contain pistons 140a, 140b and 140c (collectively referred to as pistons 140). Piston rods 141a, 141b, 141c (collectively referred to as piston rods 141) may be interconnected to or integrally formed with pistons 140 and may extend from pistons 140 and protrude from ends of cylinders 118a, 118b, 118c, respectively, and can be commonly and fixedly mounted to carriage 122. When any of pistons 140 moves within the respective cylinder 118, piston rods 141 likewise move therewith. Pistons 140 may thus be mechanically coupled to one another by piston rods 141 and carriage 122 and therefore pistons 140 can move together in unison during operation. Carriage 122 may comprise upper and lower plates 143a,b mounted to and held together by three rods 145, 147, 149. Sucker rod 110 may be mounted to carriage 122 by one or more clamps 142 or other suitable attachment mechanisms. Sucker rod 110 may also be provided with a rotator 144. Rotator 144 may rotate sucker rod 110 to promote even wearing of parts down well shaft 108, particularly in applications in which at least a portion of well shaft 108 is horizontal. Further, down-well pump 106 may resist rotation, so that rotation of sucker rod 110 may serve to tighten threaded joints, which may help to prevent disconnection of components by unthreading. Rod 149, clamp 142 and rotator 144 may be attached to one another by conventional fixation devices or techniques, with rod 149 attached to plate 143a and rotator 144 attached to plate 143b, so that collectively, rod 149, clamp 142 and rotator 144 are mounted to and connect plates 143a,b. Frame 120 may be mounted to wellhead 102 by conventional attachment devices, such as by a series of bolted and/or welded flanges.

Down-well pump 106 may be operated by reciprocating motion of pistons 140 and piston rods 141. During an upstroke, pistons 140, carriage 122, rotator 144, sucker rod 110 and down-well pump 106, hereinafter referred to collectively as the reciprocating masses, are drawn upwardly, as is down-well pump 106. Conversely, during a downstroke, the reciprocating masses are lowered. Therefore, as used herein, an upstroke or downstroke of lift system 100 means an upstroke or downstroke of the reciprocating masses. Also, the terms upstroke and downstroke refer to the movements of the pistons 140 and piston rods 141 in their cylinders 118.

By way of example only, cylinders 118 may be approximately 174 inches in length, and pistons 140 and piston rods 141 may be moved through a stroke approximately 168 inches in length. In some example embodiments, the stroke of the reciprocating masses may be between 48 inches and 360 inches in length and cylinders 118 may be slightly longer than the stroke to avoid “bottoming out” of the piston at either end of the chambers. In other embodiments, cylinders 118 may be longer if a longer stroke is desired.

Each of cylinders 118 may be a piston-type device. Each of cylinders 118a, 118b, 118c may have an upper (blind end) chamber 119a, 119b, 119c (collectively, chambers 119) above the respective one of pistons 140 and a bottom (rod end) chamber 121a, 121b, 121c (collectively, chambers 121) below the respective one of pistons 140. Each of pistons 140 has a rod end face partially defining the respective rod end chamber 121 and a blind end face partially defining the respective blind end chamber 119.

Ports 125a, 125b, 125c allow for communication of fluid into or out of each of blind end chambers 119a, 119b, 119c. Ports 125a, 125b are open to the atmosphere, while port 125c is connected to line 138. Ports 127a, 127b, 127c allow for communication of fluid into or out of each of blind end chambers 121. Ports 127a, 127b are connected to lines 146a, 146b respectively. Port 127c is connected to line 138.

The rod end inlet/outlet ports 127a, 127b of cylinders 118a, 118b, respectively are connected by fluid communication lines 146a, 146b to valve subsystem 134. Lines 146a, 146b may, by way of example only, be hoses or pipes with an internal diameter (“ID”) of 0.75 inches. Lines 146a and 146b may merge into a common line that may have an ID of 1 inch which runs into valve subsystem 134. The blind end inlet/outlet ports 125a, 125b of cylinders 118a, 118b may be open to atmosphere as the upper chambers in cylinders 118a, 118b are not filled with, and emptied of, driving fluid.

The rod end inlet/outlet 127c of cylinder 118c may be connected by fluid communication line 138 to counterbalance reservoir 136. The blind end inlet/outlet 125c of cylinder 118c may be connected by fluid communication line 148 to valve subsystem 134. In the depicted embodiment, each of lines 138 and 148 may be a hose or pipe with an ID of 1.25 inches.

Turning now to FIG. 3, example components of lift system 100 are depicted schematically in detail.

In one embodiment, valve subsystem 134 may comprise a 4-port, 3-state valve 150, check valve 152 and counterbalance valve 154. 3-state valve 150 may have a first port 151c connected to driving fluid pump 128 by way of fluid communication line 130 to receive pressurized driving fluid. A second port 151d of 3-state valve 150 may be connected to driving fluid reservoir 126 by way of fluid communication line 132 for draining driving fluid. Third and fourth ports 151a, 151b of 3-state valve 150 may be connected to two different flow paths through valve subsystem 134. While valve 150 is in a first state, lift system 100 is in a first mode of operation performing an upstroke. While valve 150 is in a second state, lift system 100 is in a second mode of operation performing a downstroke. While valve 150 is in a third state, lift system 100 may be stationary. As will become apparent, valve subsystem 134 controls flow of driving fluid, selectively directing pressurized fluid either to rod end chambers 121a,b of cylinders 118a,b or to the blind end chamber 119c of cylinder 118c.

A first flow path through valve subsystem 134 connects one port 151a of 3-state valve 150 to a first inlet/outlet 153a of valve subsystem 134 which may be in fluid communication with chambers 121a, 121b by way of lines 146a, 146b. This flow path passes through a one-way check valve 158 in one direction, and through a relief valve 156 in the other direction. Relief valve 156 may be biased closed to resist flow up to a certain pressure, which may be adjustable. In some embodiments, relief valve 156 and check valve 158 for example may be part of a model CBEA counterbalance valve, manufactured by Sun Hydraulics Corporation. Relief valve 156 may be provided with pilot lines 160, 162. Under certain conditions as will be explained below, if pressure in the blind end chamber 119c of cylinder 118c exceeds the opening pressure of relief valve 156, pilot line 162 cause relief valve 156 to smoothly open, diverting driving fluid to reservoir 126.

A second flow path through valve subsystem 134 connects one port 151b of 3-state valve 150 to a second inlet/outlet 153b of valve subsystem 134 which may be in fluid communication with chamber 119c by way of line 148.

A one-way cross-relief valve 152 lies between the first and second flow paths. The cross-relief valve may be biased closed by a pilot line 164. In some embodiments, cross-relief valve 152 may be a model COFA pilot-closed check valve, manufactured by Sun Hydraulics Corporation. Alternatively, cross-relief valve 152 may be biased closed by other means, such as electronically using a solenoid or with a spring.

In some embodiments, the components of valve subsystem 134 may be part of a single module, such as a model YDEC-LHN pressure sensitive regenerative valve assembly, made by Sun Hydraulics Corporation.

Pump 128 may be in communication with driving fluid reservoir 126 and one of ports 151a, 151b of 3-state valve 150 to supply pressurized driving fluid to cylinders 118 by way of valve subsystem 134. As will become apparent, the upstroke lifting force of lift system 100 may be created by pressure in the rod ends 121a, 121b of cylinders 118a and 118b, which may be assisted during at least part of the stroke by an upward counterbalance force acting on piston 140c. Thus, cylinders 118a, 118b are upward driving cylinders. The total lifting force acting on pistons 140a, 1′40b is the product of the pressure in the rod-end chambers during lifting and the total area of the rod-end sides of pistons 140a, 140b. Pump 128 may be therefore selected to develop sufficient pressure to provide the desired lifting force and to provide a sufficient flow rate for the desired lifting rate.

Similarly, the downstroke lowering force may be created by pressure in the blind end chamber 119c of cylinder 118c. Thus, cylinder 118c is a downward driving cylinder.

Pump relief valve 166 may provide a bypass flow path from pump 128 to fluid reservoir 126 to prevent excessive pressure from developing in the fluid communication lines, for example, when pistons 140 reach the limit of their travel. Pump relief valve 166 may be biased closed, for example by a spring. If pressure reaches a predetermined threshold, a pilot line may cause pump relief valve 166 to open, draining excess fluid back to reservoir 126.

The rod end of cylinder 118c may be in fluid communication with a counterbalance reservoir 136 by way of communication line 138. Counterbalance reservoir 136 may have two chambers, one containing a first counterbalance fluid and the other a second counterbalance fluid. In the depicted embodiment, the first counterbalance fluid may be hydraulic fluid and the second counterbalance fluid may be counterbalance fluid gas which may be nitrogen most commonly at pressures in the range of 1200-2000 psi, depending on the characteristics of the particular application, such as the size of down-hole pump 106, depth of well shaft 108 and other well characteristics. However, in some example embodiments, the second counterbalance fluid may be at pressures as low as 200 psi or as high as 3000 psi. The temperature of the second counterbalance fluid is generally close to that of ambient air in the environment in which system 100 is installed. Typically, the ambient temperature is between about −40° C. and 50° C. The second counterbalance fluid may be another suitable inert compressible fluid. A floating piston 170 separates the two chambers. During each downstroke of lift system 100, some of the first counterbalance fluid from the lower chamber of cylinder 118c and line 138 may be forced into reservoir 136, causing piston 170 to be displaced upwards to thereby compress the second counterbalance fluid (e.g. nitrogen gas), storing energy. During each upstroke, the compressed second counterbalance fluid, which is pushing against piston 170, will force the first counterbalance fluid out of reservoir 136 and into the rod end of cylinder 140c, thereby assisting lifting.

Counterbalance reservoir 136 may optionally have one or more auxiliary tanks 172a, 172b, 172c, 172d (collectively, tanks 172) of second counterbalance fluid to provide additional counterbalance fluid capacity. The additional capacity provided by tanks 172a-d keeps the relative change in volume of the second counterbalance fluid small for a given displacement of driving fluid from chamber 121c. This likewise limits the relative change in pressure of the second counterbalance fluid, so that the upward force generated by the second counterbalance fluid is approximately constant.

Controller 200 may be operable to control the operating modes or operation/state of lift system 100. Controller 200 may be electronically connected to pump 128 and to valve 150. Controller 200 may further includes a user interface with one or more control inputs (not shown). Controller 200 may also control any additional electrically operated valves in other embodiments.

Controller 200 may be, for example, a PLUS+1 MC050-10 programmable logic controller made by Sauer Danfoss. Controller 200 may also be provided with a user interface comprising a screen, such as a DP600 graphical terminal made by Sauer Danfoss. Optionally, controller 200 may also be provided with a network gateway to allow remote access to controller 200, for example, over the internet. Such a network gateway may be, for example, a PLUS+1 RG150 remote connectivity gateway made by Sauer Danfoss.

In the depicted embodiment, the respective components of valve sub-system 134 and pump 128 and the operating state thereof may be controlled either by a signal from controller 200, and/or by pressure at one or more points of lift system 100. Alternatively or additionally, the components may be individually controllable using electronic or mechanical controls at each respective component.

Turning now to FIGS. 4-7, the operation of lift system 100 will now be described.

In FIG. 4, lift system 100 is depicted in an idle state. Driving fluid is present in both the rod end and blind end chambers of cylinder 118c and the rod end chambers of cylinders 118a, 118b. Pump 128 may be idle. If pump 128 is running, valve 150 directs flow of driving fluid from pump 128 to reservoir 126. In the idle state of system 100, cross-relief valve 152 and relief valve 156 are biased to open at a line pressure of subtantially 0 psig. Thus, in the idle state of lift system 100, driving fluid may drain to reservoir 126 and driving fluid in the system may be maintained at a pressure of substantially 0 psig, with the exception of the first counterbalance fluid that is in the rod end chamber of cylinder 118c and/or counterbalance reservoir 136.

FIG. 4 depicts lift system 100 in an idle state after cylinders 118 have been primed with hydraulic driving fluid, that is, after the rod end chamber of each cylinder and the blind end chamber of cylinder 118c have been loaded with driving fluid at approximately 0 psig. Suitable procedures for priming cylinders 118 will be readily apparent to skilled persons and accordingly are not described in detail herein. In some embodiments, it may not be necessary to prime cylinders 118. Instead, air may be bled off from fluid circuit 124 over one or more strokes.

In the idle state of lift system 100, piston 140c sits in equilibrium with counterbalance reservoir 136. In particular, the weight of the reciprocating masses pulls piston 140c down. The pressure of the second counterbalance fluid in reservoir 136 tends to urge piston 170 downwards, pressurizing the first and second counterbalance fluids in counterbalance reservoir 136 and the first counterbalance fluid in the rod end chamber 121c of cylinder 140c.

Other forces may act on lift system 100 in the upwards direction. For example, as previously described, sucker rod 110 may extend through a column of oil in well shaft 108 and may have some buoyancy in the column. Friction, including friction acting on the reciprocating masses, may resist movement. Other factors which may influence the forces acting on lift system 100 may include reservoir pressure, oil viscosity, weight of the column of oil above down-well pump 106, orientation of well shaft 108, and the condition fluid circuit 124 including the presence of waxy deposits that may form in the fluid communication lines. The magnitudes of these other forces acting on system 100 may vary over the course of a stroke.

The system will reach a stable equilibrium at the point when the upward-acting forces exerted on piston 170 equal the downward-acting forces exerted on piston 170 and when the upward force of first counterbalance fluid acting on piston 140c equals the weight of the reciprocating masses. FIG. 5 depicts the forces acting on carriage 122 in the equilibrium point of system 100 in its idle state. An upward force F_c acts on piston 140c, and the weight of the reciprocating masses pulls downwardly. Other forces such as buoyancy may act upwardly. For simplicity, weight is depicted net of other upward forces acting on the reciprocating masses, such as buoyancy and friction. That is, weight is depicted net of upward-acting forces that are not exerted on the reciprocating masses by pistons 140 (referred to herein as net weight). At equilibrium, F_c will equal the net weight W of the reciprocating masses. If both faces of piston 170 have equal area, as in the depicted embodiment, this equilibrium point will occur when the pressure of the first counterbalance fluid within counterbalance reservoir 136 is equal to the pressure of the second counterbalance fluid in counterbalance reservoir 136.

The point at which system 100 will reach a stable equilibrium is dependent on design of counterbalance reservoir 136. For example, the dimensions of reservoir 136 and the faces of piston 170, and the quantity and pressure of second counterbalance fluid in reservoir 136 and any auxiliary tanks 172 in communication with reservoir 136 will determine both the equilibrium position and the pressure of the first and second counterbalance fluids at equilibrium, and at the upper and lower limits of the stroke of piston 140c.

Counterbalance reservoir may be designed so that the equilibrium point is approximately in the middle of the stroke of piston 140c. The second counterbalance fluid can be pressurized such that its minimum pressure, occurring at the top of a stroke of system 100, is at least high enough that the upward force on piston 140c is equal to the downward force on piston 140c due to unpressurized fluid in the blind end chamber of cylinder 118c. In alternate embodiments, it may be desired to have an equilibrium point that is not mid-stroke. In such embodiments, the equilibrium point could be moved up in the stroke by putting more of the second counterbalance fluid in reservoir 136 and tanks 172, that is, increasing the pressure of the second counterbalance fluid in reservoir 136 and tanks 172, or it could be moved down in the stroke by removing some second counterbalance fluid from the reservoir and reducing pressure in reservoir 136 and tanks 172, that is, reducing the pressure of counterbalance fluid in reservoir 136 and tanks 172.

To begin cycling lift system 100 from an idle state, pump 128 may be activated using a control module of controller 200. Cross relief valve 152 and relief valve 156 can be first biased closed and 3-state valve 150 put in its upstroke state.

In FIG. 6, system 100 is depicted during an upstroke. Three-state valve 150 is in its first (upstroke) state. Pump 128 provides pressurized driving fluid, which travels through check valve 158 of valve subsystem 134 and to the rod ends of cylinders 118a, 118b by way of lines 146a, 146b. Cross-relief valve 152 is biased closed by pilot line 164. In alternate embodiments where cross-relief valve 152 is not hydraulically piloted, it may instead be biased to open above a certain pressure, so that it is normally closed but may open to relieve excess pressure.

The pressurized fluid increases pressure in the rod end chambers of cylinders 118a, 118b and urges pistons 140a, 140b upwards. The blind end chambers of cylinders 118a and 118b are filled with air and are open to the atmosphere. As pistons 140a, 140b are urged upwards, air is expelled from the blind ends of cylinders 118a, 118b.

Piston 140c is likewise urged upwards by virtue of being mechanically coupled to pistons 140a, 140b by carriage 122.

Upward pressure is also exerted on piston 140 by the counterbalance fluid, through piston 170 and the first counterbalance fluid in reservoir 136 and the rod end chamber of cylinder 118c. As piston 140c progresses upwardly, the second counterbalance fluid is allowed to expand and pressure in counterbalance reservoir 136 decreases, as does the upward force exerted on piston 140c.

Over the entire upstroke, on average, the effect of the counterbalance reservoir 136 offsets at least part of the weight of the reciprocating masses. That is, the counterbalance subsystem urges piston 140c upwards with a force equal to a substantial portion of the weight of the reciprocating masses. In the depicted embodiment, the volume of second counterbalance fluid in counterbalance reservoir 136 and auxiliary tanks 172 is large relative to the change in volume due to compression during a stroke of lift system 100. As will be appreciated, the change in pressure, and thus, the change in upward force on piston 140c over a stroke may be relatively small. In some embodiments, the maximum second counterbalance fluid pressure, occurring at the bottom of a stroke, may be no more than 8%-15% higher than the minimum second counterbalance fluid pressure, occurring at the top of a stroke.

As piston 140c travels upwards, first counterbalance fluid is expelled from the blind end of cylinder 118c and flows to valve subsystem 134 by way of communication line 148. While 3-state valve 150 is in its first (upstroke) state, fluid is free to drain from communication line 148 to reservoir 126.

FIG. 7 depicts the forces acting on carriage 122 during an upstroke. Forces F_a and F_b, act on pistons 140a, 140b respectively, and are approximately equal to the pressure in the rod end chambers 121a, 121b of cylinders 118a, 118b, multiplied by the areas of pistons 140a, 140b on their rod-end faces, that is, the areas of the pistons 140a, 140b, less the areas of the rods 141a, 141b themselves. Another upward force F_c acts on piston 140c and is equal to the pressure in chamber 121c, multiplied by the area of the rod-end face of piston 140c. The other forces acting on carriage 122, such as weight, buoyancy and friction, are assumed to act in a net downward direction and are depicted as F_o. As the pistons 140 travel upwardly, the volume of the rod end chambers 121a, 121b increases at a rate equal to the linear speed of the pistons, multiplied by the total area of the rod-end faces of pistons 140a, 140b. Thus, pump 128 should be capable of providing fluid at sufficient pressure to generate the desired upward force and at a sufficient flow rate to maintain the desired rate of piston travel.

It is desirable that the pistons 140 will not “bottom out” at either end of the chambers during the upstroke or downstroke. Therefore the system may be configured to alternate between the modes of operations (i.e. upstroke/downstroke) before the pistons reach the end of the chambers.

However, possibly, pistons 140 may reach the limit of their travel within cylinders 118 while 3-state valve 150 is in its first (upstroke) state or downstroke state. If pump 128 continues to run, excess pressure may develop. In the event of excess pressure, pump relief valve 166, which is normally closed, opens to provide relief. Specifically, in response to excess pressure at the outlet of pump 128, pilot line 174 causes pump relief valve 166 to open, allowing driving fluid to drain from the outlet of pump 128 back to driving fluid reservoir 126.

When an upstroke or downstroke is completed, lift system 100 may transition to a stationary state. If lift system 100 is running continuously, it may only stay in the stationary state very briefly or only momentarily or not at all. Alternatively, lift system 100 may remain in a stationary state indefinitely at the end of an upstroke or downstroke.

FIG. 8 depicts a downstroke of system 100. During a downstroke, a signal from controller 200 causes valve 150 to transition to its upstroke state. In this state, pressurized driving fluid flow from pump 128 flows from port 151b through port 153b and to the blind end chamber 119c of cylinder 118c, urging piston 140c downwards. Downward movement of piston 140c expels fluid from chamber 121c and into reservoir 136, causing second counterbalance fluid in reservoir 136 to be compressed. Thus, as piston 140c moves downwardly, it does work on the second counterbalance fluid. The energy associated with lowering the reciprocating masses is stored so that the energy can be used to assist in raising the reciprocating masses during the upstroke as described above.

Downward movement of piston 140c also urges pistons 140a and 140b downwards by virtue of their mechanical coupling at carriage 122. Downward movement of pistons 140a, 140b causes driving fluid to be expelled from the rod end chambers of cylinders 118a, 118b respectively. The expelled driving fluid flows under pressure through communication lines 146a, 146b to components of valve subsystem 134. The pressurized flow causes cross-relief valve 152 to open, allowing pressurized fluid to flow from communication lines 146a, 146b to communication line 148 and then into the blind end of cylinder 118c by way of valve subsystem 134. Fluid expelled from cylinders 140a, 140b is therefore used to supplement the flow of fluid from pump 128 to cylinder 140c. In other words, driving fluid expelled from cylinders 118a, 118b is regenerated under pressure to cylinder 118c.

As will be apparent, driving fluid flowing to valve subsystem 134 from lines 146a, 146b during a downstroke must pass through cross-relief valve 152 unless relief valve 156 is open. As previously described, relief valve 156 is normally closed, however, if excess pressure occurs in valve subsystem 134, pilot lines 160 and/or 162 may cause relief valve 156 to open, allowing excess driving fluid to drain to reservoir 126 by way of 3-state valve 150. Driving fluid that is expelled from cylinders 118a, 118b during a downstroke therefore flows into communication line 148 unless excess pressure develops, in which case, it is drained to reservoir 126.

As depicted in FIG. 6, during the upstroke of lift system 100, pressure is substantially released from driving fluid in the blind end chamber of cylinder 118c. Thus, the fluid in that chamber does not significantly resist the upstroke of pistons 140. In contrast, during the downstroke of pistons 140, driving fluid in the rod end chambers of cylinders 118a, 118b is maintained under pressure and therefore resists the downstroke.

FIG. 9 depicts the forces acting on carriage 122 during a downstroke. Pressure in counterbalance reservoir 136 causes an upward force to be exerted on piston 140c. As piston 140c progresses in a downward direction, second counterbalance fluid in counterbalance reservoir 136 and any auxiliary tanks 172 is compressed, increasing its pressure and increasing the upward force from the first counterbalance fluid acting on piston 140c. The upward force acting on piston 140c increases from a minimum at the top of the downstroke to a maximum at the bottom of the downstroke, when the second counterbalance gas is in its most highly compressed state.

On average, over the downstroke, the effect of the force produced by the counterbalance fluids and counterbalance reservoir 136 balances at least a substantial part of the weight of the reciprocating masses.

During the downstroke, a force F_c acts in the downward direction through piston 140c. Force F_c is equal to the pressure of driving fluid in the upper/blind end chamber 119c of cylinder 118c, multiplied by the area of piston 140c on its blind end face, less the upward force resulting from second counterbalance fluid in lower chamber 121c, that is the pressure in chamber 121c, multiplied by the area of the rod end face of piston 140c. Forces F_a and F_b act in the upward direction against pistons 140a, 140b respectively and are approximately equal to the products of the pressure in the rod end chambers of cylinders 118a, 118b and the areas of pistons 140a and 140b on their rod end faces. The other forces acting on carriage 122 and thus on the piston rods and pistons that are connected to the carriage 122, such as weight, buoyancy and friction, are depicted as F_o. Again, the upward force resulting from the first counterbalance fluid acting in piston 140c offsets at least part of the weight of the reciprocating components.

if the pressure in the upper/blind end chamber of cylinder 118c is equal to the pressure in the lower/rod end chambers of cylinders 118a, 118b, the area of the blind end face of piston 140c must be larger than the total area of the rod end faces of pistons 140a, 140b in order to yield a net downward force and drive pistons 140 downwardly.

In the depicted embodiment, the area of the upper/blind end face of piston 140c may be double the total area of the rod end faces of pistons 140a, 140b. With this ratio, if the driving fluid pressure in the lower/rod end chambers of cylinders 118a, 118b is equal to that in the upper/blind end chamber of cylinder 118c, the downward force acting on piston 140c, less the upward forces acting on pistons 140a, 140b will be approximately equal to the upward force acting on pistons 140a, 140b during an upstroke.

Therefore, if the pressures in the lower/rod end chambers of cylinders 118a, 118b and the upper/blind end chamber of cylinder 118c are held substantially equal throughout substantially the entire upstroke and substantially the entire downstroke, and if the net effect of the counterbalance, weight, and other down-well forces is on average the same over both upward and downward strokes, the rates of the upstroke and downstroke will be substantially the same. If the area of the blind end face of piston 140c is substantially double the total area of the rod end faces of pistons 140a, 140b, the required flow rate of driving fluid in the downstroke will be approximately twice the flow rate that is required in the upstroke. Absent regeneration of driving fluid, a larger pump flow of driving fluid would be required for the downstroke than for the upstroke.

Regeneration of driving fluid from the rod end chambers of cylinders 118a, 118b to the blind end chamber of 118c by way of valve subsystem 134 allows the larger flow rate required for the downstroke to be obtained using a relatively small pump sized to deliver the flow rate required for the upstroke. This may provide cost and/or energy efficiency benefits compared to a system which uses a larger pump.

Following completion of a downstroke, lift system 100 may begin a new upstroke as depicted in FIGS. 6-7. Thus, lift system 100 may be operated in a substantially continuous mode of operation alternating substantially continuously between an upstroke and a downstroke. Lift system 100 may also be operated in a manner where the rate of movement of the pistons 140 on the upstroke is different than the rate of movement on the downstroke. This may be achieved by having controller 200 adjust the flow rate provided by pump 128 on the upstroke compared to the downstroke.

Alternatively, following the completion of any upstroke or downstroke, or even possibly during an upstroke or downstroke, lift system 100 may be returned to the idle state depicted in FIGS. 4-5 and may remain in that state indefinitely.

FIG. 10. depicts another example lift system 300. Lift system 300 may have upward driving cylinders 318a, 318b and downward driving cylinder 318c, like cylinders 118a, 118b, 118c, with pistons 340a, 340b, 340c (collectively, pistons 340) therein. Piston rods 341a, 341b, 341c may extend from pistons 340a, 340b, 340c and protrude from cylinders 118 and may be mounted to a carriage like carriage 122 to control a sucker rod, down-well pump and possibly, other reciprocating masses, such as referenced in the embodiment of FIGS. 1-10.

System 300 may be equipped with a pump 328. The pump 328 can be chosen to provide flow rates/pump pressures that are suitable for a particular lift system 300 and application. In the depicted embodiment, pump 328 may be like pump 128, in particular, it may be a variable-displacement piston pump and may be able to deliver a maximum and constant flow rate of about 46 gallons per minute at a pressure of at least 3000 psi. For example, pump 328 may as before, be a series 45 axial piston open circuit pump made by Sauer Danfoss. Alternatively, pump 328 could be another type of pump operable to deliver suitable pressures and flow rates, such as a vane or gear pump. A piloted relief valve 366 can allow excess pressure from pump 328 to drain to reservoir 326. Valve 366 may be biased closed up to a certain pressure, which may be infinitely variable between a certain maximum and minimum.

Pump 328 may be controlled for pressure-compensated operation. That is, pump 328 can be controlled to operate so as to maintain a substantially constant pressure of driving fluid (which again may be hydraulic fluid). Run in these conditions, pump 328 may output a substantially constant volumetric flow rate of driving fluid. The pressure and flow rate output by pump 328 may be linked. That is, increasing the pressure produced by pump 328 may also result in an increased flow rate, while decreasing the pressure may result in a decreased flow rate of the driving fluid. Additionally, if the resistance to movement of driving fluid though out the driving fluid system changes over time in an upstroke or downstroke, to maintain a substantially constant flow rate during the upstroke or downstroke it may be required to adjust the pressure setting for the pump during the upstroke and downstroke by a controller such as a controller 400.

System 300 may comprise a controller 400 to control the operation of its components. Controller 400 may be, for example, a PLUS+1 MC050-10 programmable logic controller made by Sauer Danfoss. Controller 400 may also be provided with a user interface comprising a screen, such as a DP600 graphical terminal made by Sauer Danfoss. Optionally, controller 400 may also be provided with a network gateway to allow remote access to controller 400, for example, over the internet. Such a network gateway may be, for example, a PLUS+1 RG150 remote connectivity gateway made by Sauer Danfoss.

Each one of cylinders 318 has a rod (lower) and blind (upper) end. Each one of pistons 340 defines two chambers within the respective cylinder 318, with one lower chamber lying between the piston and the rod end of the cylinder (rod end chambers 321a, 321b, 321c, respectively) and one upper chamber lying between the piston and the blind end of the cylinder (blind end chambers 319a, 319b, 319c, respectively). Each one of pistons 340 has a rod end face partially defining the lower/rod end chamber and a blind end face partially defining the upper/blind end chamber.

Each one of cylinders 318 has an inlet/outlet port 325 for its upper/blind end chamber and an inlet/outlet port 327 for its lower/rod end chamber. Ports 325a, 325b are open to the atmosphere, Port 325c is connected to line 348 for supplying driving fluid to, or draining driving fluid from, upper/blind end chamber 319c. Ports 327a, 327b are connected to lines 346a, 346b for supplying driving fluid to or draining driving fluid from chambers 321a, 321b respectively. Port 327c is connected to line 338 to allow counterbalance fluid to flow between chamber 321c and counterbalance fluid tanks 372a, 372b, 372c (collectively, tanks 372).

The rod end inlet/outlet ports 327a, 327b of cylinders 318a, 318b are connected by fluid communication lines 346a, 346b to valve subsystem 334. In the depicted embodiment, lines 346a, 346b may be hoses or pipes and may have for example have an internal diameter (“ID”) of about 0.75 inches. Lines 346a and 346b merge into a common line that may be hoses or pipes that may have an ID of about 1 inch which runs into tee 333 and then a port 380 of valve subsystem 334. The blind end inlet/outlet ports 325a, 325b, 325c of cylinders 318a, 318b may be open to atmosphere as the upper chambers in cylinders 318a, 318b are not filled with, and emptied of, driving fluid.

The rod end inlet/outlet of cylinder 318c may be connected by fluid communication line 338 to, and in fluid communication with, counterbalance fluid tanks 372. The blind end inlet/outlet of cylinder 318c may be connected by fluid communication line 348 to port 382 of valve subsystem 334. In the depicted embodiment, each of lines 338 and 348 may be a hose or pipe and may, for example, have an ID of about 1.25 inches.

Valve subsystem 334 may comprise a two way pilot-operated valve 350 and a one-way pilot operated valve 352. Valve 350 may be, for example, a model RSJC8 pilot operated, balanced piston sequence valve produced by Sun Hydraulics Corporation. Valve 352 may be, for example, a model RPKC8 pilot operated, balanced piston relief valve produced by Sun Hydraulics Corporation. Valve 350 may be biased closed up to the pressure in line 360, which allows a small amount of fluid to flow from line 358, at the pressure in line 358 to line 362 and valve 354. Valve 350 may open when the pressure in pilot line 396 exceeds the pressure in line 360, which biases valve 350 closed.

Valve 354 may be, for example, a model RBAP electro-proportional relief valve produced by Sun Hydraulics Corporation. A small amount of fluid is allowed to flow through line 362 to an input and a pilot line of valve 354. A solenoid may bias valve 354 closed up to a certain pressure differential between lines 364 and line 362 (the opening pressure). If this pressure difference exceeds the opening pressure, valve 354 opens and a small amount of fluid flows through line 364 to reservoir 326 by way of port 384. The opening of valve 354 reduces the pressure in lines 360, 362, which in turn reduces the pressure to which valve 350 is biased closed. Thus, valve 354 may control valve 350. That is, valve 350 will open if the pressure in line 358 reaches the opening pressure of valve 354.

A signal from controller 400 may control the opening pressure of valve 354 by way of a solenoid. This likewise may control the pressure at which valve 350 will open. Valve 354 may be infinitely variable between a certain minimum and maximum. In the depicted embodiment, valve 354 is variable between a minimum opening pressure of 0 psi and a maximum opening pressure of about 3000 psi. When valve 354 is set to open at 0 psi, valve 350 is effectively opened. When valve 354 is set to open at 3000 psi, it closes valve 350 to pressures below 3000 psi. The path from port 384 to reservoir 326 may be designed to minimize backpressure. As valve 354 operates based on the differential pressure between lines 362 and 364, reducing backpressure in lines 364 will allow valves 350 and 354 to be opened in response to a lower input pressure at line 358.

Valve 352 may be operated in a similar way to valve 350. Valve 352 is biased closed by pressure in line 374 and piloted by pressure in line 376. Pressure in line 374 is controlled by electrically controlled valve 356. Valve 352 may open when the pressure in pilot line 376 exceeds the pressure in line 374, which biases valve 352 closed. Valve 356 may be, for example, a model RBAP electro-proportional relief valve produced by Sun Hydraulics Corporation.

Lines 374, 378 may permit a small amount of driving fluid to flow from line 370, at the fluid pressure in line 370. A small amount of fluid at this pressure is allowed to flow through line 374 to an input and a pilot line of valve 356. A solenoid biases valve 356 closed up to a certain pressure differential between lines 378 and line 379 (the opening pressure). If this pressure difference exceeds the opening pressure, valve 354 opens and a small amount of fluid flows through line 380 to reservoir 326 by way of port 384. The opening of valve 356 reduces the pressure in lines 370, 378, which in turn reduces the pressure to which valve 352 is biased closed. Thus, valve 356 may control valve 352. That is, valve 352 will open if the pressure in line 370 reaches the opening pressure of valve 356.

A signal from controller 400 may control the opening pressure of valve 356 by way of a solenoid. This may likewise control the pressure at which valve 352 will open. Valve 356 may be infinitely variable between a certain minimum and maximum. In the depicted embodiment, valve 356 is variable between a minimum opening pressure of 0 psi and a maximum opening pressure of about 3000 psi. When valve 356 is set to open at 0 psi, valve 352 is effectively opened. When valve 356 is set to open at 3000 psi, it closes valve 352 to pressures below 3000 psi.

Valve subsystem 334 may also have ports 388 and 390 for measuring fluid pressure in lines 358, 370 respectively. Fluid cannot flow through ports 388, 390. Rather, ports 388 and 390 provide a pressure reading.

It should be noted that lines 360, 362, 374, 378 may be provided with restrictors to limit the amount of driving fluid that can flow therethrough.

Much like system 100, system 300 as illustrated has a counterbalance subsystem. In system 300, rod end chamber 321c of cylinder 318c is filled with a pressurized counterbalance fluid. Rod end port 327c of chamber 321c is in communication by way of line 338 with a counterbalance fluid reservoir comprising 3 tanks 372 of pressurized counterbalance fluid that may be a compressible inert gas. Nitrogen gas may be the counterbalance fluid. However, in other embodiments, the counterbalance gas may be another compressible inert gas. Cylinder 318c with piston 340c may be selected to accommodate this configuration being designed with among other things appropriate seals and be made from appropriate materials that allow the piston 340c to maintain driving fluid such as hydraulic fluid in the upper chamber and nitrogen gas in the lower chamber and may be able to sustain continued operation for a prolonged period of time without encountering a significant degree of cylinder failure during operation. Pressurized counterbalance gas in chamber 321c urges piston 340c upwards, offsetting the weight of the reciprocating masses, in much the same way as the counterbalance of lift system 100 described above.

Counterbalance gas in chamber 321c and tanks 372 is typically pressurized to between 1200 and 2000 psi and may be approximately at typical environment temperatures in the range of about −40° C. to 50° C. The amount of pressure of the counterbalance gas is determined by the weight of the reciprocating masses. For heavier reciprocating masses, counterbalance gas will be more highly pressurized. In some embodiments, the pressure of the counterbalance gas may be as low as 200 psi or as high as 3000 psi or possibly higher.

In some embodiments, counterbalance fluid may be sufficiently pressurized to offset about 60% of the weight of the reciprocating masses. It has been found that in some applications, this results in an equilibrium point near the middle of a stroke of system 300, which typically results in sufficient efficiency levels. The pressure of counterbalance fluid in chamber 321c and tanks 372 varies over each stroke of system 300, with a pressure minimum occurring at or near the top of the stroke and a pressure maximum near the bottom of the stroke. Of course, the amount of pressure variation depends on the total volume of counterbalance fluid in tanks 372 and chamber 321c—the larger the volume of counterbalance fluid, the smaller the variation in pressure.

The operation of lift system 300 will now be described with reference to FIGS. 11-12.

To begin cycling lift system 300 from an idle state, pump 328 is activated using controller 400.

In FIG. 11, system 300 is depicted during an upstroke. Valves 354 and 350 are in their closed state, that is, they are biased closed to a pressure exceeding the normal operating pressure expected at port 380. Pump 328 provides pressurized driving fluid, which travels through check valve 392 and to the rod ends of cylinders 318a, 318b by way of lines 346a, 346b.

The pressurized fluid increases pressure in the lower/rod end chambers of cylinders 318a, 318b and urges pistons 340a, 340b upwards. The upper/blind end chambers of cylinders 318a and 318b are filled with air and are open to the atmosphere. As pistons 340a, 340b are urged upwards, air is expelled from the blind ends of cylinders 318a, 318b through ports 325a, 325b.

Piston 340c is likewise urged upwards by virtue of being mechanically coupled to pistons 340a, 340b by a carriage which is connected to the sucker rod etc.

Upward pressure is also exerted on piston 340c by the counterbalance fluid in chamber 321c. Other upward forces may also act on the system, such as buoyancy of the sucker rod in the well and friction in the well may also act against the direction of movement.

As piston 340c progresses upwardly, the counterbalance fluid is allowed to expand and pressure in chamber 321c and tanks 372 decreases, as does the upward force exerted on piston 340c.

Over the entire upstroke, on average, the effect of the pressurized counterbalance fluid in chamber 321c may at least partially offset the weight of the reciprocating masses, less any other upward-acting forces. In the depicted embodiment, the volume of counterbalance fluid in chamber 321c and auxiliary tanks 372, may be large relative to the change in volume due to compression during a stroke of lift system 300. As will be appreciated, the change in pressure, and thus, the change in upward force on piston 340c over a stroke may be relatively small. In some embodiments, the peak counterbalance fluid pressure, occurring at the bottom of a stroke, may be no more than 8%-15% greater than the minimum pressure, occurring at the top of a stroke. For practical purposes, the upward force may be considered to be substantially constant over a stroke in either the up or down direction, and is equal to a portion of the weight of the reciprocating masses. In some embodiments, the upward force from counterbalance fluid in chamber 321c is equal to approximately 60% of the weight of the reciprocating masses. In other embodiments, counterbalance fluid pressure may be tuned to produce a different average upward force. For different applications, e.g. different wells, pumps designs or sucker rod designs, different counterbalance forces will yield the optimum efficiency. The desired counterbalance force for a particular application may be, for example, experimentally determined.

As piston 340c travels upwards, driving fluid is expelled from the blind end of cylinder 318c and flows to valve subsystem 334 by way of communication line 148. During the upstroke, controller 400 causes valves 352, 356 to be substantially open to 0 psi, which freely allows driving fluid to drain from communication line 348 to reservoir 326 by way of port 382, valve 352 and port 386.

During an upstroke of system 300, the forces acting on the carriage of system 300 vary substantially as described above with respect to system 100, and as illustrated in FIG. 7.

It is desirable that the pistons 340 will not “bottom out” at either end of the chambers during the upstroke or downstroke. Therefore the system may be configured to alternate between the modes of operations (i.e. upstroke/downstroke) before the pistons reach the end of the chambers.

Possibly, however, pistons 340 may reach the limit of their travel within cylinders 318 while valve subsystem 334 is in its first (upstroke) state, that is, while valves 350, 354 are closed to a predetermined opening pressure and valves 352, 356 are substantially open. In that event, the volume of the rod end chambers of cylinders 318a, 318b can no longer increase. If pump 328 continues to run, excess pressure may develop at port 380 and in line 358. In the event of excess pressure, valves 354 and 350, which are normally closed, open to provide relief. Specifically, in response to excess pressure at port 380 and in line 358, valves 354 and 352 open, allowing driving fluid to drain to reservoir 326 by way of ports 384 and 386, thereby relieving excess pressure.

FIG. 12 depicts a downstroke of system 300. During a downstroke, a signal from controller 400 causes valves 350, 354 to be set to open to substantially 0 psi. Pressurized driving fluid provided by pump 328 flows to port 380 and through line 358, valve 350 and port 382 to the blind end chamber of cylinder 318c, urging piston 340c downwards. Downward movement of piston 340c compresses counterbalance fluid in chamber 321c and tanks 372. Thus, as piston 340c moves downwardly, it does work on the counterbalance fluid. The energy associated with lowering the reciprocating masses is stored so that the energy can be used to assist in raising of the reciprocating masses during the upstroke as described above.

Downward movement of piston 340c also urges pistons 340a and 340b downwards by virtue of their mechanical coupling. Downward movement of pistons 340a, 340b causes driving fluid to be expelled from the rod end chambers of cylinders 318a, 318b respectively. The expelled driving fluid flows under pressure through communication lines 346a, 346b to port 380 of valve subsystem 334.

During the downstroke, valves 350, 354 are opened substantially to 0 psi. Pressurized driving fluid flows from lines 346a, 346b to port 380 and through line 358, valve 350 and port 382 to the blind end chamber of cylinder 318c. Thus, fluid expelled from cylinders 340a, 340b is added to flow from pump 328, effectively doubling the flow rate of fluid being supplied to chamber 319c. In other words, fluid expelled from cylinders 340a, 340b is regenerated under pressure to cylinder 340c. At the same time, controller 400 causes valves 352, 356 to close to approximately 3000 psi, preventing the driving fluid flowing into valve subsystem 334 at port 380 from flowing through valve 352 to port 386 and reservoir 326.

If excess pressure occurs in valve subsystem 334, pilot lines 374, 378 may cause relief valves 356, 352 to open, allowing excess driving fluid to drain to reservoir 326 by way of port 382. Driving fluid that is expelled from cylinders 318a, 318b during a downstroke therefore flows into rod end chamber 319c of cylinder 340c unless excess pressure develops, in which case, at least some of the fluid is drained to reservoir 326.

As in system 100, in system 300 during the downstroke of pistons 340, driving fluid in the lower/rod end chambers of cylinders 318a, 318b is maintained under pressure and therefore resists the downstroke.

During the downstroke, counterbalance fluid in chamber 321c and tanks 372 causes an upward force to be exerted on piston 340c. As piston 340c progresses in a downward direction, counterbalance fluid in chamber 321c and tanks 172 is compressed, increasing its pressure and increasing the upward force on piston 340c. The upward force increases from a minimum at the top of the downstroke to a maximum at the bottom of the downstroke, when the counterbalance gas is in its most highly compressed state. On average, over the downstroke, the effect of the counterbalance fluid balances at least part of the weight of the reciprocating masses.

During the downstroke, the forces acting on the carriage of lift system 300 vary substantially as illustrated in FIG. 9 and described above for lift system 100.

The area of the blind end face of piston 340c may be substantially double the total area of the rod end faces of pistons 340a, 340b. With this ratio, if the driving fluid pressure in the rod end chambers of cylinders 318a, 318b is substantially equal to that in the blind end chamber of cylinder 318c, the downward force acting on piston 340c, less the upward force acting on pistons 340a, 340b, will be approximately equal to the upward force acting on pistons 340a, 340b during an upstroke

Therefore, if the pressures in the rod end chambers 321a, 321b of cylinders 318a, 318b and the blind end chamber 319c of cylinder 318c are held equal throughout substantially the entire upstroke and substantially the entire downstroke, the rates of the upstroke and downstroke will be approximately the same. As the area of the blind end face of piston 340c is double the total area of the rod end faces of pistons 340a, 340b, the required flow rate of driving fluid in the downstroke will be approximately twice the flow rate that is required in the upstroke. Absent regeneration of driving fluid, a larger pump or at least a greater flow rate by a pump would therefore be required for the downstroke than for the upstroke.

Regeneration of driving fluid from the rod end chambers of cylinders 318a, 318b to the blind end chamber of 318c by way of regeneration subsystem 334 allows the larger flow rate required for the downstroke to be obtained using a relatively small pump sized to deliver the flow rate required for the upstroke. This may provide cost and/or energy efficiency benefits compared to a system which uses a larger pump.

Following completion of a downstroke, lift system 300 may begin a new upstroke as depicted in FIG. 11.

Conveniently, providing valves 366, 350/354, 352/356 that are infinitely variable within a certain pressure range allows for the speed of the upstroke and downstroke of system 300 to be tuned within a certain interval. By varying the flow rate provided by pump 328 (with corresponding changes in pump pressure) the upper pressure settings for the valve pairs 350/354 and 352/356 can be adjusted through controller 400 to accommodate lower or increased pressures thus allowing for slower or faster speeds of travel of the pistons 340a-c in the cylinders 318a-c.

As described above, the state of lift systems 100, 300 are controlled by controllers 200, 400, respectively. Specifically, controller 200 controls the state of pump 128 and 3-state valve 150, thus controlling the state of lift system 100, and controller 400 controls the state of pump 328 and valves 354, 356, thereby controlling the state of lift system 300.

After the completion of a stroke, system 100 or system 300 may be maintained in a stationary state. However, it will sometimes be desirable to run system 100 or system 300 substantially continuously. In such a case, system 100 or system 300 may be in a stationary state only very briefly, during the transition from upstroke to downstroke or vice-versa.

Controllers 200, 400 may be interconnected with one or more sensors to detect the position of pistons 140 or pistons 340, respectively. As depicted in FIGS. 3, 4, 6, 8-9, controller 200 may be interconnected with a linear position sensor 202 located on cylinder 118b. Linear position sensor 202 may be for example an LK series transducer made by Rota Engineering. Sensor 202 may be able to detect the movement of the piston rod 141b and/or piston 140b and output a signal indicative of the linear position of piston 140b within cylinder 118b. Sensor 202 may also output a signal indicative of the velocity of piston 140b.

When the signal from sensor 202 indicates that piston 140b is approaching the top of an upstroke, controller 200 may respond to this signal by causing 3-state valve 150, and thus, lift system 100, to change from its upstroke state to its downstroke state. When the signal from sensor 202 indicates that piston 140b is approaching the bottom of a downstroke, controller 200 may respond by causing 3-state valve 150 and thus, lift system 100 to transition from its downstroke state to its upstroke state.

As depicted in FIGS. 10-12, controller 400 may be interconnected with a linear position sensor 402, like linear position sensor 202, located on cylinder 318b. Sensor 402 may output a signal indicative of the position of piston 340b within cylinder 318b. Sensor 402 may also output a signal indicative of the velocity of piston 340b.

When the signal from sensor 402 indicates that piston 340b is approaching the top of an upstroke, controller 400 may respond by transitioning lift system 300 to a downstroke state. Specifically, controller 400 may first cause the opening pressure of valves 354, 350 to gradually lower. This will cause valve 350 to slowly open, allowing some driving fluid to flow from port 380, through valves 350 and 352 to reservoir 326. The flow rate of driving fluid into chambers 321a, 321b will therefore decrease until it reaches zero at the very peak of the upstroke, when valve 350 is fully open. The upward velocity of pistons 340 will likewise decrease until it reaches zero at the top of the upstroke.

At the top of the upstroke, pistons 340 may be above the equilibrium point at which the counterbalance fluid in chamber 321c is sufficient to support the reciprocating masses. As a result, when valve 350 is fully open, pistons 340 may momentarily begin to fall, as pressure will be momentarily released from chambers 321a, 321b. Sensor 402 may be configured to indicate that piston 340b is beginning to fall, and in response, controller 400 will cause valves 356, 352 to gradually close, diverting flow from pump 328 and from chambers 321a, 321b to port 382 and out to chamber 319c. Once valves 356, 352 are fully closed, pistons 340 will be driven downwardly at their full downstroke speed. Thus, at the top of an upstroke, pistons 340 gradually decelerate, briefly stop moving, and then gradually accelerate downwards to the downstroke speed.

When the signal from sensor 402 indicates that piston 340b is approaching the bottom of a downstroke, controller 400 may respond by transitioning lift system 300 back to an upstroke state. Specifically, controller 400 may first cause the opening pressure of valves 352, 356 to gradually lower. This will cause valve 352 to slowly open, allowing some driving fluid to flow from port 380, through valves 350 and 352 to reservoir 326. The flow rate of driving fluid into chamber 319c will therefore decrease until it reaches zero at the very bottom of the downstroke, when valve 352 is fully open. At the same time, controller 400 may cause the opening pressure of valves 354, 350 to gradually increase, effectively slowly closing those valves. The gradual closing of valve 350 may have a braking effect, decelerating the reciprocating masses. The downward velocity of pistons 340 may therefore decrease until it reaches zero at the bottom of the downstroke.

At the bottom of the downstroke, pistons 340 may be below the equilibrium point at which the counterbalance fluid in chamber 321c is sufficient to support the reciprocating masses. As a result, when valve 352 is fully open, pistons 340 may momentarily begin to rebound upwardly, as pressure may be momentarily released from chambers 319c. Sensor 402 may be configured to indicate that piston 340b is beginning to rise. Controller 400 may cause valves 354, 350 to gradually close, directing flow from pump 328 into chambers 321a, 321b. Pistons 340 may therefore gradually accelerate upwardly. Once valves 356, 352 are fully closed, pistons 340 may be driven upwardly at their full upstroke speed. Thus, at the bottom of a downstroke, pistons 340 gradually decelerate, briefly stop moving, and then gradually accelerate upwards to the upstroke speed.

Alternatively, if desired, controller 400 may cause system 300 to transition to an idle state. In such a case, controller 400 may be set to cause valves 352, 356 to open, allowing driving fluid pressure to drain from the system. Controller 400 may cause valves 350, 354 to gradually open, so that the system gradually returns to equilibrium in its idle state awaiting the beginning of the next stroke. Pump 328 may also be powered down while system 300 is in its idle state.

It may sometimes be desired to hold the reciprocating masses stationary in a position other than the equilibrium position. This may be effected by, for example, manual override at the user interface of controller 400. In response, controller 400 may cause valves 352, 356 to open, allowing driving fluid to drain to reservoir 326. Meanwhile, valves 350, 354 may be held closed by biasing valve 354 closed up to a high pressure. As check valve 392 prevents fluid from flowing back to pump 328, driving fluid may therefore be held under pressure in chambers 321a, 321b, holding pistons 340 in position.

Sensors 202, 204 and 402, 404 thus enable controllers 200 and 400 to control the period and transition between the upward and downward strokes of system 100 and system 300, respectively. As desired, systems 100, 300 may be run continuously, with substantially no delay between successive strokes, or they may be run intermittently.

Alternatively or additionally, position sensors may be provided on cylinders 118a, 118c, 318a, 318c, pistons 140, 340, or other components of lift systems 100, 300. Sensors may also be provided on any of the valves of valve system 134 or valve subsystem 334, to detect the presence of excess pressure. Excess or a predetermined pressure may indicate, for example, that the end of a stroke is near or has been reached.

Signals from sensors 202, 204 and 402, 404 may also be used to control the speed of the strokes of lift systems 100, 300. For example, controller 200 or controller 400 may measure the elapsed time between the beginning and end of each stroke. If the elapsed time is higher than desired, controller 200 or controller 400 may then output a signal to pump 128 or pump 328, respectively, to increase the flow rate, increasing the stroke speed. Conversely, if the elapsed time is lower than desired, controller 200 or controller 400 may then output a signal to pump 128 or pump 328, respectively, to decrease the flow rate, decreasing the stroke speed. Also, through speed measurement sensors the controller may be able to monitor the speed of movement in approximately real time and make appropriate adjustments to the speed of movement of the pistons during the strokes.

In some applications, it may be desired to have the upstroke and downstroke occur at substantially constant, but different speeds. For example, in heavy oil pumping applications, it may be desired to perform downstrokes slowly, to allow oil to flow into the down-well pump. When it is desired to run the stokes at different rates, an entry may be made at the user interface of controller 400, in response to which controller 400 sends signals to cause pump 328 to run at a first flow rate and pressure during one stroke and at a second flow rate and pressure during the other stroke.

As described above, in different applications, lift systems 100 and 300 may be operated with varying quantities of counterbalance fluid. As will be appreciated, for a given lift system, increasing the amount of counterbalance fluid will shift the equilibrium point upwards in the stroke. That is, the point at which the counterbalance fluid is sufficient to support the reciprocating masses without assistance from driving fluid in chambers 121a, 121b or 321a, 321b will be shifted upwards by adding counterbalance fluid.

Of course, if the amount of counterbalance fluid is altered, the pressure of the counterbalance fluid, and therefore, the upward force exerted on piston 140c or 340c at different points in the stroke will likewise be altered. Any efficiency benefit to be gained from the counterbalance is therefore dependent on appropriate tuning of counterbalance fluid pressures, that is, selection of an appropriate quantity/pressure of counterbalance fluid.

The appropriate amount/pressure of counterbalance fluid will vary from application to application and from well to well and may be influenced by factors such as weight of the reciprocating masses, sizes of various components of the lift system, depth of the well, fluid column height and composition within the well, sucker rod construction, and numerous other factors. Accordingly, the optimum counterbalance tuning will typically need to be experimentally determined for each well, and may need to be periodically adjusted over the life cycle of the well.

In some embodiments, the most efficient counterbalance configuration will be the configuration that results in the lift system expending approximately equal amounts of energy and producing approximately equal peak forces during the upstroke and the downstroke. This may be determined by estimating an appropriate quantity of counterbalance fluid (measured, e.g. by pressure of counterbalance fluid at a particular point in a stroke), and cycling lift system 100 or 300 while measuring with some kind of suitable electrical power/current sensor the power consumed by pump 128 or pump 328. The information of power consumption may be provided to the controller so that an operator or the controller can make suitable adjustments. If the power consumed by pump 128 or pump 328 is higher during a downstroke than during an upstroke, it will tend to indicate excess pressure of counterbalance fluid. In such a case, counterbalance fluid should be released from the system and the experiment repeated. Conversely, if the power consumed by pump 128 or pump 328 is higher during an upstroke, it will tend to indicate insufficient counterbalance fluid pressure, meaning that fluid should be added to the system. In general, once the power is consistent through both strokes, the counterbalance will be correctly tuned. Of course, as well conditions may change over time, this counterbalance tuning process should be repeated periodically to maintain desired efficiency levels.

Lift systems 100, 300 may be configured in a range of sizes for driving different loads. Dimensions for 3 example systems are set out in table 1, below. The three systems, designated “standard”, “heavy” and “super lift”, are intended for loads of increasing size. Of course, the dimensions contained in table 1 are by way of example only and other embodiments could be configured in a range of different sizes for other applications.

	TABLE 1
	Std	Heavy	Super Lift
Peak operating pressure	3000	psi	3000	psi	3000	psi
Piston 140a~b/340a-b	2.5	in	2.5	in	3.5	in
diameter
Rod 141a-b/341a-b diameter	2	in	1.5	in	2	in
Total lifting area of pistons	3.53	in2	6.28	in2	12.96	in2
140a-b/340a-b
Estimated lift capacity of	10,603	lb	18,850	lb	38,877	lb
pistons 140a~b/340a-b
Piston 140c/340cdiameter	3	in	4	in	5.75	in
Rod 141c/341c diameter	1.25	in	1.5	in	1.5	in
Total lifting area of piston	5.84	in2	10.80	in2	24.20	in2
140c/340c
Lowering area of piston	7.07	in2	12.57	in2	25.97	in2
140c/340c
Estimated lift capacity	17,524	lb	32,398	lb	72,600	lb
of piston 140c/340c
Estimated total lift capacity	28,127	lb	51,248	lb	111,477	lb
of pistons 140/340

Variations of the illustrated embodiments are contemplated. By way of example only, instead of having the three cylinders 118a, 118b and 118c or 318a, 318b, 318c being horizontally aligned, with the downward driving and/counter balance cylinder being located between the two upward driving cylinders, the following are example variations:

In one variation, the orientation of cylinders 118/318 may be reversed, with piston rods 141/341 extending upwardly from cylinders 118/318 to a carriage above cylinders 118/318.

In another variation, system 300 may be provided with one or more pressure relief lines between rod end chambers 321a,b and/or blind end chamber 319c and reservoir 326. By way of example, the pressure relief lines may run from fluid communication lines 346a,b to reservoir 326. Each pressure relief line may pass through one or more relief valves, which may be normally biased closed up to a high fluid pressure (for example, 3000 psi) and which may be electrically or hydraulically controlled to open in the event of excess pressure developing in rod end chambers 321a,b and or blind end chamber 319c. With the pressure relief valves closed, fluid may be prevented from draining through the pressure relief lines to reservoir 326. When the pressure relief valves open in the event of excess pressure, fluid may drain to reservoir 326, relieving the excess pressure. One or more pressure relief valves may thus be provided that if the pressure gets too high, allow for pressure to be relieved in the rod end chambers 321a,b and lines 346a, 346b during the downstroke when pistons 340a, 340b are providing regeneration hydraulic fluid for blind end chamber 319c of cylinder 318c.

When introducing elements of the present invention or the embodiments thereof, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements.

Of course, the above described embodiments are intended to be illustrative only and in no way limiting. The described embodiments of carrying out the invention are susceptible to many modifications of form, arrangement of parts, details, and order of operation. The invention, therefore, is intended to encompass all such modifications within its scope.

Claims

1. A hydraulic lift system comprising:

a source of pressurized hydraulic fluid;

a plurality of hydraulic cylinders, each one of said cylinders having a piston therein with a piston rod of said piston extending from an end of each one of said hydraulic cylinders, wherein said piston rods are mechanically interconnected so that said piston rods and said pistons of each of said hydraulic cylinders are operable to move upwards and downwards in unison with each other;

a hydraulic fluid communication sub-system operable to deliver fluid from said source of pressurized hydraulic fluid to at least a first cylinder of said plurality of hydraulic cylinders to drive said pistons in an upward direction through an upstroke; said hydraulic fluid communication sub-system also operable to deliver hydraulic fluid from said source of pressurized hydraulic fluid to a second cylinder of said plurality of hydraulic cylinders to drive said pistons in a downward direction through a downstroke; said hydraulic fluid communication sub-system also operable to deliver hydraulic fluid from said first cylinder to said second cylinder;

a hydraulic fluid flow control sub-system operable to: (a) selectively direct hydraulic fluid from said source of pressurized hydraulic fluid to said first cylinder to drive said pistons in an upward direction to provide an upstroke; and (b) alternatively, selectively direct hydraulic fluid from said source of pressurized hydraulic fluid to said second cylinder to drive said pistons in a downward direction to provide a downstroke; and (c) during said downstroke direct hydraulic fluid from said first cylinder to said second cylinder to assist said second cylinder in driving said pistons in said downward direction during said downstroke.

2. A system as claimed in claim 1, wherein said second cylinder has an upper chamber and a lower chamber separated by a common piston, and wherein said hydraulic fluid is operable to be communicated to said upper chamber to provide a force from above said piston during said downstroke.

3. A system as claimed in claim 2 wherein said piston rods both extend from a lower end of their respective cylinders and wherein said piston rods are operable to be interconnected to a reciprocating mass.

4. A system as claimed in claim 3 wherein said reciprocating mass comprises a sucker rod operable to be interconnected to a down-well pump in a well.

5. A system as claimed in claim 2 further comprising a counterbalance subsystem operable to supply a pressurized counterbalance fluid to said lower chamber of said second cylinder, said counterbalance fluid operable to provide a force from below said piston in said second cylinder during said upstroke and said downstroke, said counterbalance fluid counteracting at least some the gravitational force associated with said reciprocating mass.

6. A system as claimed in claim 5, wherein said counterbalance fluid comprises a pressurized inert gas.

7. A system as claimed in claim 6 wherein said gas is nitrogen.

8. A system as claimed in claim 6, wherein said counterbalance system comprises a reservoir operable for storing a supply of said pressurized inert gas.

9. A system as claimed in claim 8 wherein said reservoir is in communication with said lower chamber of said second cylinder.

10. A system as claimed in claim 5, wherein said counterbalance sub-system comprises:

a counterbalance cylinder with a counterbalance piston therein defining a hydraulic fluid chamber containing a hydraulic fluid and a counter balance fluid chamber, containing a pressurized inert gas,

a fluid communication line connecting said hydraulic fluid chamber in fluid communication with said second hydraulic cylinder, such that said pressurized fluid and said counterbalance piston exert pressure on said hydraulic fluid in said hydraulic fluid chamber, urging said piston of said second hydraulic cylinder in an upward direction.

11. A system as claimed in claim 5, wherein a reciprocating mass is interconnected to said piston rods of said first and second cylinders and wherein said system is operable such that said reciprocating mass is driven upwards and downwards by said upstroke and said downstroke of said piston rods of said first and second cylinders, and wherein said counterbalance subsystem is operable to urge said piston of said second hydraulic cylinder upwardly with a force substantially equal to a substantial portion of the gravitational force acting downwards associated with said reciprocating mass.

12. A system as claimed in claim 1, wherein said hydraulic fluid flow control sub-system comprises a three-state valve being operable to selectively direct said hydraulic fluid from said first cylinder to said second cylinder to assist said second hydraulic in driving said pistons in said downward direction during said downstroke.

13. A system as claimed in claim 12, wherein said hydraulic fluid flow control sub-system comprises one or more relief valves.

14. A system as claimed in claim 13, wherein said one or more relief valves comprises at least one hydraulically piloted valve.

15. A system as claimed in claim 14, wherein said one or more relief valves comprises at least one valve that is electrically biased to a closed.

16. A system as claimed in claim 1 further comprising a controller operable for controlling the operation of said hydraulic fluid flow control sub-system.

17. A system as claimed in claim 16 wherein said source of pressurized hydraulic fluid comprises a pump and further wherein said controller is operable for controlling the operation of said pump.

18. A system as claimed in claim 1, wherein said second cylinder has an upper chamber for receiving said driving fluid such that said second cylinder is operable to drive said piston of said second cylinder downwardly and said at least one first cylinder has a lower chamber for receiving said driving fluid such that said first cylinder is operable to drive said piston of said first cylinder upwardly.

19. A system as claimed in claim 1, wherein:

said plurality of hydraulic cylinders comprises said first and second cylinders and further comprises a third cylinder;

said hydraulic fluid communication sub-system is operable to deliver fluid from said source of pressurized hydraulic fluid to both said first cylinder and said third cylinders such that said first and third cylinders are operable to drive said pistons in an upward direction through an upstroke; and

said hydraulic fluid flow control sub-system is operable to: (a) selectively direct hydraulic fluid from said source of pressurized hydraulic fluid to said first cylinder and said third cylinder to drive said pistons in an upward direction to provide an upstroke; and (b) alternatively, selectively direct hydraulic fluid from said source of pressurized hydraulic fluid to said second cylinder to drive said pistons in a downward direction to provide a downstroke; and (c) during said downstroke direct hydraulic fluid from said first cylinder and said third cylinder to said second cylinder to assist said second cylinder in driving said pistons in said downward direction during said downstroke.

20. A system as claimed in claim 19 wherein said second cylinder is positioned between said first and third cylinders.

21. A system as claimed in claim 20 wherein said second cylinder is transversely aligned between said first and third cylinders.

22. A system as claimed in claim 21, wherein said second cylinder has an upper chamber for receiving said driving fluid for driving said piston of said second cylinder downwardly.

23. A system as claimed in claim 22, wherein each of said first and third cylinders has a lower chamber for receiving said driving fluid for driving said pistons of said first cylinder and third cylinders upwardly.

24. A system as claimed in claim 18 wherein a cross-sectional area of said upper chamber in said second cylinder is approximately double the cross-sectional areas of said lower chamber in each of said first and third cylinders.

25. A system as claimed in claim 1, wherein said piston rods are interconnected to a sucker rod of a well pump, and wherein said system is operable to operate said well pump in a shaft of a well.

26. A system as claimed in claim 21, wherein said piston rods are interconnected to a sucker rod of a well pump, and wherein said system is operable to operate said well pump in a shaft of a well.

27. A system as claimed in claim 26, wherein said second cylinder is axially aligned with said sucker rod and wherein said first and third cylinders are transversely spaced a substantially equal distance on either side of said second cylinder whereby rotational forces acting on said sucker rod can be substantially eliminated.

28. A system as claimed in claim 27 wherein said piston rods of each of said first, second and third cylinders are interconnected to a common carriage and said common carriage is interconnected to said sucker rod.

29. A system as claimed in claim 1 wherein said hydraulic fluid flow control sub-system comprises a plurality of valve devices interposed between said plurality of cylinders and said source of pressurized hydraulic fluid.

30. A system as claimed in claim 29 wherein plurality of valve devices comprises at least one electronically operated valve device.

31. A system as claimed in claim 30 further comprising a controller, and wherein said controller is operable to control said at least one electronically operated valve device.

32. A system as claimed in claim 1 further comprising a controller operable to control at least a component of said hydraulic flow control sub-system.

33. A system as claimed in claim 1 further comprising a reservoir for said hydraulic fluid, said reservoir being in fluid communication with said hydraulic fluid communication sub-system, such that hydraulic fluid may be communicated to said reservoir.

34. A system as claimed in claim 33 wherein said hydraulic flow control sub-system is operable to direct the flow of hydraulic fluid to said reservoir from said second cylinder during said downstroke.

35. A system as claimed in claim 32 further comprising a reservoir for said hydraulic fluid, said reservoir being in fluid communication with said hydraulic fluid communication sub-system, such that hydraulic fluid may be communicated to said reservoir.

36. A system as claimed in claim 35 further comprising a controller operable to control at least a component of said hydraulic flow control sub-system, and wherein said controller is operable to control the flow of hydraulic fluid to said reservoir from said second cylinder during said downstroke.

37. A method of reciprocating a down-well pump in a shaft of a well, said method comprising:

a) pumping a pressurized fluid into a lift chamber of a first hydraulic cylinder to lift a carriage coupled to said down-well pump and to a piston of said first hydraulic cylinder;

b) pumping a pressurized fluid into a lowering chamber of a second hydraulic cylinder having a piston coupled to said carriage, to lower said carriage;

c) connecting said lift chamber in fluid communication with said lowering chamber such that pressurized fluid is expelled from said lift chamber into said lowering chamber during said lowering.

38. A method as claimed in claim 37, said method further comprising:

urging said piston of said second hydraulic cylinder in an upward direction using an inert gas counterbalance cylinder to offset at least a portion of the weight of said carriage, said down-well pump and masses reciprocated therewith.

39. A lift system comprising

a pump for supplying a flow of pressurized driving fluid;

at least one upward driving cylinder having a movable piston rod;

at least one downward driving cylinder having a movable piston rod; said piston rods of said upward driving cylinder and said downward driving cylinder being interconnected to each other such that said piston rods of both said upward driving cylinder and said downward driving cylinder are operable to move upwards and downwards in unison with each other;

a driving fluid communication sub-system operable to deliver a flow of driving fluid supplied by said pump from said pump to said upward driving cylinder to drive said piston rods in an upward direction in an upstroke;

said driving fluid communication sub-system also operable to deliver a flow of driving fluid supplied by said pump from said pump to said downward driving cylinder to drive said piston rods in a downward direction in a downstroke; and said driving fluid communication sub-system also operable to deliver a flow of driving fluid in said upward driving cylinder from said upward driving cylinder to said downward driving cylinder during said downstroke;

a fluid direction control sub-system operable to: (a) in a first mode of operation to direct a flow of driving fluid from said pump to said upward driving cylinder to drive said pistons in an upward direction to create an upstroke; (b) in a second mode of operation to direct a flow of driving fluid from said pump to said downward driving cylinder to drive said pistons in a downward direction to create a downstroke; and (c) in said second mode of operation, to also direct a flow of driving fluid from said upward driving cylinder to said downward driving cylinder during said downstroke, such that during said downstroke, said driving fluid is delivered from said upward driving cylinder to/towards said downward driving cylinder to assist said downward driving cylinder in driving said pistons in said downward direction during said downstroke.

40. A method of moving a reciprocating mass upwards and downwards, said method comprising

a) providing a pump for supplying a flow of pressurized driving fluid;

b) providing at least one upward driving cylinder having a movable piston rod interconnected to said reciprocating mass;

c) providing at least one downward driving cylinder having a movable piston rod interconnected to said reciprocating mass; said piston rods of said upward driving cylinder and said downward driving cylinder being interconnected to each other such that said piston rods of both said upward driving cylinder and said downward driving cylinder are operable to move upwards and downwards in unison with each other;

d) providing a driving fluid communication sub-system for delivering a flow of driving fluid supplied by said pump from said pump to said upward driving cylinder to drive said piston rods in an upward direction in an upstroke and for delivering a flow of driving fluid supplied by said pump from said pump to said downward driving cylinder to drive said piston rods in a downward direction in a downstroke, said driving fluid communication sub-system also for delivering a flow of driving fluid in said upward driving cylinder from said upward driving cylinder towards said downward driving cylinder during said downstroke;

e) providing a fluid direction control sub-system;

f) directing a flow of driving fluid from said pump to said upward driving cylinder to drive said pistons in an upward direction to create an upstroke to thereby move said reciprocating mass upwards;

g) directing a flow of driving fluid from said pump to said downward driving cylinder to drive said pistons in a downward direction to create a downstroke; and simultaneously also directing a flow of driving fluid from said upward driving cylinder to said downward driving cylinder during said downstroke, such that during said downstroke, said driving fluid is delivered from said upward driving cylinder to said downward driving cylinder to assist said downward driving cylinder in driving said pistons in said downward direction during said downstroke, to thereby move said reciprocating mass downwards.

41. A method of operating a lift system, wherein lift system comprises:

a pump for supplying a flow of pressurized driving fluid;

at least one upward driving cylinder having a movable piston rod:

at least one downward driving cylinder having a movable piston rod; said piston rods of said upward driving cylinder and said downward driving cylinder being interconnected to each other such that said piston rods of both said upward driving cylinder and said downward driving cylinder are operable to move upwards and downwards in unison with each other;

a driving fluid communication sub-system for delivering a flow of driving fluid supplied by said pump from said pump to said upward driving cylinder to drive said piston rods in an upward direction in an upstroke and for delivering a flow of driving fluid supplied by said pump from said pump to said downward driving cylinder to drive said piston rods in a downward direction in a downstroke, said driving fluid communication sub-system also for delivering a flow of driving fluid in said upward driving cylinder from said upward driving cylinder towards said downward driving cylinder during said downstroke;

a fluid direction control sub-system operable to: (a) in a first mode of operation to direct a flow of driving fluid from said pump to said upward driving cylinder to drive said pistons in an upward direction to create an upstroke; (b) in a second mode of operation to direct a flow of driving fluid from said pump to said downward driving cylinder to drive said pistons in a downward direction to create a downstroke; and (c) in said second mode of operation, to also direct a flow of driving fluid from said upward driving cylinder to said downward driving cylinder during said downstroke;

and wherein said method comprises:

a) directing a flow of driving fluid from said pump to said upward driving cylinder to drive said pistons in an upward direction to create an upstroke;

b) directing a flow of driving fluid from said pump to said downward driving cylinder to drive said pistons in a downward direction to create a downstroke; and simultaneously also directing a flow of driving fluid from said upward driving cylinder to said downward driving cylinder during said downstroke, such that during said downstroke, said driving fluid is delivered from said upward driving cylinder to said downward driving cylinder to assist said downward driving cylinder in driving said pistons in said downward direction during said downstroke.